Method for introducing carbon into chemical vapor deposited refractory metal,and resulting product

ABSTRACT

CONVENTIONAL CHEMICAL VAPOR DEPOSITION TECHNIQUE IS EMPLOYED TO INTRODUCE A TRACE AMOUNT OF CARBON INTO A REFRACTORY METAL, SUCH AS TUNGSTEN, FOLLOWED BY HEAT TREATMENT OF THE METAL, TO STABILIZE THE GRAIN STRUCTURE,   STRENGTHEN THE METAL, AND PREVENT GROWTH OF THE GRAINS WHEN THE METAL IS SUBJECTED TO A HIGH TEMPERATURE ENVIRONMENT.

March 30, 1971 HOLZL ETAL METHOD FOR INTRODUCING CARBON INTO CHEMICAL VAPOR DEPOSITED REFRACTORY METAL, AND RESULTING PRODUCT 5 Sheets-Sheet 1 Filed April 1, 1968 FIG.

2 P0854? AF /3% P/c/meo D. HAHN BY ATTORNEY March 30, 1971 HOLZL ETAL 3,573,092

METHODJFQR INTRODUCING CARBON INTO CHEMICAL VAPOR DEPOSITED REFRACTORY METAL, AND RESULTING PRODUCT Filed April 1, 1968 3 Sheets-Sheet 2 3 .35 36 7 34 a a: a ii B-I/Z 39 4/ 42 40 2 Q4 9 1e .1 W a f Q 2 m 8 VAcuuM PuMP INVENTORS Fl 6.4 55355? @5256 BY M ATTORNEY 39, 1%?! R HQLZL ET AL 3,573,692

METHOD FOR INTRODUCING CARBON INTO CHEMICAL VAPOR DEPOSITED REFRACTORY METAL. AND RESULTING PRODUCT Filed April 1, L968 .3 Sheets-Sheet 8 .58 57 9 7/ 6/ O 1/ V V .WF l n 4 73 74 N N K r v 63 5 /7:? J 54"" INVENTORS PoaaerAJ/azzz. BY 2/0/490 .0. f/n/wv United States Patent Ofice 3,573,092 Patented Mar. 30, 1971 3,573,092 METHOD FOR INTRODUCING CARBON INTO CHEMICAL VAPOR DEPOSITED REFRAC- TORY METAL, AND RESULTING PRODUCT Robert A. Holzl, La Canada, and Richard D. Hahn, Canoga Park, Calif., assignors to Fansteel Inc., North Chicago, Ill.

Filed Apr. 1, 1968, Ser. No. 718,019 Int. Cl. C23c 11/00 U.S. Cl. 117-1072. 12 Claims ABSTRACT OF THE DISCLOSURE Conventional chemical vapor deposition technique is employed to introduce a trace amount of carbon into a refractory metal, such as tungsten, followed by heat treatment of the metal, to stabilize the grain structure, strengthen the metal, and prevent growth of the grains when the metal is subjected to a high temperature environment.

DISCLOSURE This invention relates to the deposit of refractory metals on a mold or substrate by chemical vapor deposition (hereinafter designated as CVD), and more particularly, to the deposit of such metal in the presence of a gaseous carbon source, to effect dispersion of a trace amount of carbon in the CVD metal to improve the physical properties of the thus deposited metal.

BACKGROUND OF THE INVENTION CVD is a well known process comprising reduction of a gaseous compound of a substance, such as a metal, in the presence of a reducing agent to cause deposition of the metal onto a hot surface of a substrate or mold. It is frequently employed for the formation of refractory metal coatings on the substrate; and it is characteristic of such refractory metal deposit that a columnar grain structure is inherently formed. This columnar structure is not all to be desired insofar as strength properties are concerned.

In the case of pure tungsten, which is a commonly employed refractory metal for various purposes, such as crucibles for conducting various chemical reactions, and heat-resistant tubing desirable for use for the conduction of liquids and gases under elevated temperature environments of up to about 2400 0., grain growth of the columnar structure occurs at such elevated temperatures, which weakens the metal.

SUMMARY AND OBJECTS The instant invention is designed to overcome such problem of grain growth of CVD metals, especially tungsten. Pursuant to this invention it has been found that the grain growth can be obviated by introduction of a trace amount of carbon dispersed substantially uniformly throughout the metal, by effecting the carbon deposit from a gaseous source thereof in the presence of the gaseous reactants from which the metal deposits on the hot surface. By effecting the carbon deposit from a gaseous source of carbon in the presence of the CVD reactants, delicate control of the amount of carbon can be obtained. This would otherwise not be the case in introducing carbon into the metal by a metallurgical sintering method wherein a fine powder of metal and carbon are compressed together and sintered at a high temperature.

As will be discussed later in greater detail, this is so because it is critical in the invention hereof that not too much carbon be deposited with the tungsten, not more than about 800 parts per million (p.p.m.) by weight. With a sintering method, such trace amount of discrete carbon powder cannot be introduced uniformly into the metal.

After the CVD dispersion of carbon throughout the CVD deposited tungsten is effected, the resultant tungsten with a trace amount of carbon is heat treated, which converts the columnar grain structure to a strong equiaxed grain structure wherein grain growth does not occur. In this connection, an equiaxed grain structure is one in which the grains have approximately the same dimensions in all directions, as is defined in the American Society of Metals handbook, 8th Edition, 1961, on page 15, published at Menlo Park, Ohio. The time and temperature of the heat treatment are relatively immaterial as long as sufiicient heat energy is imparted to cause grain transformation from columnar to a strong equiaxed structure.

The invention hereof is not dependent on particular vapor deposition temperatures, as the usual temperatures commonly employed in CVD are applicable herein. It is only necessary that the deposition on the hot surface of the mold or substrate be effected in the presence of a carbon-containing gas which will result in uniform dispersion of carbon in the metal. Conventional vapor deposition techniques comprise depositing the metal onto the hot surface, by reaction of a gaseous compound of the metal usually in the gaseous halogen state such as the hexafluoride, with a reducing agent such as hydrogen. These reactants are caused to flow under a vacuum into a reaction chamber which contains the hot surface of the object to be plated.

Carbon dioxide gas is an advantageous source of carbon employed herein because it is easier to control such gas for obtaining the requisite trace amount of carbon in the metal, than other gaseous carbon sources, such as hydrocarbon gases, for example, methane and pentane. However, hydrocarbon gases can be functionally employed.

From the preceding, it is seen that the invention has as its objects, among others, the provision of an improved apparatus and method for forming a strong stable equiaxed grain structure from columnar deposited CVD refractory metals, especially tungsten, by uniform dispersion of a trace amount of carbon in the metal, which are simple and economical, and to an improved CVD metal deposit wherein grain growth is obviated. Other objects of the invention will become apparent from the following more detailed description and accompanying drawings in which:

DETAILED DESCRIPTION FIG. 1 is a photomicrograph of a typical CVD pure tungsten deposit, illustrating the inherent columnar grain structure thereof in as deposited condition power magnification);

FIG. 2 is a photomicrograph of CVD tungsten containing 400 parts per million of carbon dispersed therein from carbon dioxide, in as deposited condition (200 power magnification);

FIG. 3. is aphotomicrograph of the tungsten-400 ppm. carbon deposit of FIG. 2 after heat treatment at 2,000 C. for one hour, illustrating the transformation to a strong stable equiaxed grain structure (500 power magnification) FIG. 4 is a schematic sectional elevation of a conventional unidirectional CVD system, which may be employed for effecting the carbon dispersion of this invention; and

FIG. 5 is a similar schematic sectional elevation illustrative of another type of CVD system which may be employed, namely, a reverse flow CVD system of the type ilustrated in assignees copending application by Frederick A. Glaski and James R. Humphrey, Ser. No. 645,- 278, filed June 12, 1967, for Methods and Apparatus for Vapor Deposition.

The instant invention has been found particularly advantageous in improving markedly the physical properties of CVD tugsten. Hence, it is described in detail with reference to tungsten. As previously explained, the physical CVD method employed for effecting the tungsten deposit is immaterial, as any suitable method may be utilized. All that is necessary is to introduce the carboncontaining gas into the reaction chamber simultaneously with introduction of the reactants for effecting deposit of the tungsten.

Usually such reactants are tungsten hexafiuoride as the gas, and hydrogen gas as the reducing agent. Conventional deposition of the metal on the hot surface of the mold or substrate is usually effected at a temperature of 600 to 700 C. At such temperature, the gaseous source of carbon, advantageously carbon dioxide, introduced into the CVD reaction chamber simultaneously with the other reactants, is reduced and carbon is uniformly dispersed throughout the tungsten as it deposits on the hot surface.

As for the physical methods that can be employed, unidirectional flow of the gases through the reaction chamber is suitable. However, it is advantageous to utilize the physical methods dislosed in assignees copending application by Robert A. Holzl, Frederick A. Glaski and James R. Humphrey, Ser. No. 620,164, filed Mar. 2, 1967, now abandoned, for Vapor Deposition Method and Apparatus and in assignees aforementioned application, Ser. No. 645,278 (illustrated in FIG. for the reason that both these methods obviate tapered plating on the mold or substrate, which obtains in the unidirectional flow method.

In the method of application, Ser. No. 620,164, the tungsten is deposited in extremely thin layers by rapidly filling the CVD reaction chamber with the-reactants-at such high velocity, while pocketing the reactants therein, as to cause each layer to be substantially uniformly deposited over the entire surface of the hot mold or substrate. In the latter application and as illustrated in FIG. 5 hereof, the method is adapted particularly for relatively long objects, and is effected by forming the layers in such manner that they are each tapered but the flow of the reactants is alternately reversed through the reaction chamber to cause all adjacent layers to taper in opposite directions to thus compensate for the taper of the respective layers, and provide an overall coating or plating of substantially uniform thickness. Another suitable physical method is disclosed in US. Pat. 3,031,388, dated Apr. 24, 1962.

With respect to the amount of carbon dispersed in the tungsten, if too much carbon is present, a substantial quantity of tungsten carbide will be formed which results in the tungsten becoming extremely weak and brittle, and is hence undesirable. Also, too much carbon prevents transformation of the tungsten to a strong and stable equiaxed grain structure upon heat treatment. If too little carbon is dispersed, then the effect thereof is not obtained. The lower amount of carbon should, therefore, not be much below about 150 p.p.m. by weight of the total amount of tungsten and the carbon; and it should not be much above about 800 ppm. Carbon content is readily determined by conventional combustion analysis.

The carbon dispersion in the deposited tungsten in such minute or trace quantity, can be readily obtained by controlling the amount (rate of flow) of the carbon-containing gas with reference to the amount of tungsten hexafluoride from which the tungsten is deposited on the hot surface of the mold. The amount of carbon dioxide introduced into the reaction chamber will vary with the surface area being plated but it will generally be in the range of about 5 to 25% by volume standard (standard temperature and pressure conditionsSTP) of the volume of tungsten hexafluoride standard, to obtain a carbon con tent in the tungsten of about 150 to 800 ppm. In this connection, the reduction of carbon dioxide to carbon is 4 not so efiicient that all the carbon dioxide is reduced. Consequently a relatively large volume of carbon dioxide escapes from the reaction chamber.

Within such range of the trace amount of carbon, the hardness of the tungsten-carbon dispersion hereof is substantially the same as the hardness of a pure CVD tungsten deposit, thus evidencing that there is substantially no significant tungsten carbide formation which would otherwise result in substantial increase in hardness and brittleness. In this respect, pure CVD tungsten deposits have a hardness in the range of about 350 to 450 DPH (diamond pyramid hardness test) on the Vickers scale, while the tungsten-carbon dispersion hereof within the carbon content range noted, has a hardness of about 400 to 500 DPH.

As previously related, after the tungsten-carbon dispersion has been deposited on the mold, it is heat treated, desirably while on the mold, to transform the columnar structure to the equiaxed grain structure. The heat treatment is effected in any suitable non-oxidizing atmosphere, such as a pure inert gas but preferably under a high vacuum in the order of 10* torr (absolute pressure) or less. After the heat treatment, the dispersion is then separated from the mold; in the case of a steel mold by pulling it off the mold or in the case of a mold, such as molybdenum, by dissolving the mold in a strong acid, such as nitric acid, which does not attack tungsten. However, the coating of the tungsten-carbon dispersion on the mold, can be first removed, and then heat treated.

Regarding the character of the heat treatment, it is only necessary to heat for a suflicient time and at a sufficient temperature until transformation to the equiaxed grain structure is effected. The transformation is not reversible; and after such transformation, it is immaterial how high the temperature is raised or the length of time heat is applied, as long as the tungsten-carbon dispersion is not heated to the extent where incipient fusion or melting occurs. As a precautionary measure, it is desirable not to exceed a heat treatment temperature of about 2400 C. because at about this temperature the tungsten-carbon dispersion may deteriorate. In this connection, the tungstencarbon dispersion is stable (no grain growth) up to about such temperature of 2400 C., which is substantially the maximum temperature environment in which CVD tungsten articles are employed. A suitable time and temperature for the heat treatment is about 60 to 15 minutes at a temperature of 1800 C. to 2200 C. It is not desirable to heat below the minimum indicated as then the transformation to the equiaxed state may not occur.

The heat treatment is desirably effected in a furnace because this provides a slow cooling rate when the heat treatment is terminated, which enhances precipitation of carbon at the tungsten grain boundaries thus promoting grain stability. However, other means, such as electrical resistance heating, may be employed.

As is noted in the description of the drawings, FIG. 1 illustrates a typical columnar grain structure of a pure CVD tungsten deposit. FIG. 2 illustrates a CVD tungsten deposit hereof containing 400 ppm. carbon uniformly dispersed throughout the tungsten, and in as deposited condition (before heat treatment). It will be noted that the structure is not as definitely columnar as the pure tungsten deposit because of the trace amount of carbon uniformly dispersed intragranularly. Upon heat treatment, as illustrated in FIG. 3, the tungsten carbon dispersion of FIG. 2 is converted to a strong, fine equiaxed grain structure which is defined by the carbon precipitate at the grain boundaries. Without the presence of the trace amount of carbon, grain growth of the pure tungsten of FIG. 1 will occur, markedly under high temperature environments, which results in weakening of the structure.

By the presence of the carbon in the trace amounts noted, namely, about to 800 p.p.m., the yield strength is increased by a factor of at least 25 of about 1400 C.,

compared to pure columnar tungsten deposits. Yield strength is the force required to deform the metal permanently. At 1400 C., pure columnar deposited tungsten has a yield strength of about 25,000 pounds per sq. inch, while the equiaxed tungsten-carbon dispersion of this in vention has a yield strength at such temperature of at least about 30,000 to 35,000 pounds per sq. inch, and may run as high as 50,000 pounds per sq. inch; and as previously explained, the equiaxed structure is stable at temperatures up to about 2400 C. At room temperature, the yield strength of conventional columnar CVD tungsten is about 50,000 pounds per sq. inch compared to the equiaxed structure of this invention which has a yield strength of at least about 100,000 and as high as 150,000 pounds per sq. inch, an increase by a factor of at least 2.

The following are illustrative of typical CVD systems, and typical examples illustrative of the invention, wherein a CVD deposit of tungsten is obtained in a reaction chamber through which the reactants are caused to flow under a vacuum which for tungsten is usually about to 29 inches of mercury (vacuum). Referring to FIG. 4, it illustrates a conventional form of apparatus wherein the method of this invention can be conducted by the conventional unidirectional flow of the reactants.

The apparatus comprises conventional glass tubing 2 of about 2 inches inside diameter and 36 inches long, usually of quartz or Vycor glass, removably mounted on a support base 3 to form a conventional deposition reaction chamber. Inasmuch as a. vacuum is applied to the reaction chamber in the deposition or plating process, a peripheral seal 6 of suitable material, such as a rubber gasket, is provided in a groove in base 3 to engage over the lower end of tube 2 and seal the same.

A work holder 7, provided with a disc-shaped end 8 is removably mounted on such base; and desirably, although not essential, work holder 7 is rotated by suitable mechanism (not shown) about an upright axis so that during the deposition process, all of the surface of the object to be plated will be uniformly exposed to the reactants. At its upper end, work holder 7 is provided with a horizontal support table 9 for holding the object 11 to be coated, which in the embodiment illustrated is a mold or mandrel in the form of a short molybdenum rod. When plated on its exterior surface, the mandrel forms a hollow crucible which is subsequently removed.

An induction heating coil 12 is conventionally positioned about tube 2 at the zone of mandrel 11 to impart the requisite heat to the mandrel for effecting vapor deposition of tungsten on the exterior thereof. A conventional rubber stopper 13 is removably secured in the top end of tube 2; and a vacuum line tubing 14 is connected to a vacuum pump 17 which provides a constant vacuum source. A suitable vacuum gauge 18 is connected in line 14-.

In accordance with conventional vapor deposition technique, a source 21 of tungsten hexafiuoride (WF is connected to tubing 22 by means of tubing 23 in which is connected a conventional flow meter 24 and a manually operable control valve 26 to control the meter for presetting its rate of flow. Tubing 22 provides a line for conducting reactants into the reaction chamber 4, and is removably gripped in stopper 13. A source 27 of hydrogen (H is also connected to tubing 22 by means of tubing 28 and a manifold tube 29 connected to tubing 23 beyond valve 26. A conventional flow meter 32 and manually operable control valve 33 are also provided to control the hydrogen flow in line 28. Also connected to tubing 22 through manifold 29 and tubing 34 is a source 36 of an inert purging gas, desirably argon (A), with another conventional flow meter 37 and a manually controlled valve 38 connected in tubing 34. Finally, means is provided for introduction of carbon dioxide (CO from source 39- into reaction chamber 4 by means of inlet tubing 40 connected to manifold 29, and having 6 therein a conventional flow meter 41 and manually operable control valve 42.

Except for the valve controlled means for introduction of the carbon dioxide, the system is conventional for all systems in which there is unidirectional flow of the reactants effected by vacuum pump 17; and as is conventional in such systems, the work is heated up to a desired temperature by induction heating coil 12, and the reactant gases and carbon dioxide are caused to flow continuously through reaction chamber 4 at the desired quantity (rate). Before the deposition process is initiated, the reaction chamber is desirably flushed with argon and after such flushing, the supply of argon is shut off. Although FIG. 1 illustrates tungsten hexafluoride as one of the reactants for plating of mold 11 with tungsten, it can be any other gaseous tungsten compound, such as any gaseous halide of tungsten, but tungsten hexafluoride is preferred.

Example I The following is a typical operating example for the plating of mold 11 with the tugnsten-carbon dispersion hereof to form a crucible of essentially tungsten having an equiaxed grain structure.

First of all, with vacuum pump 17 operating, and all valve closed except the argon control valve, argon is caused to flow through the reaction chamber at 2 standard liters per minute as measured by flow meter 37. The vacuum is shut off, and the argon pressure in the chamber is allowed to rise to approximately 1 atmosphere whereupon the vacuum is again applied to the chamber and argon is again allowed to flow into the chamber to repeat the flushing cycle. This flushing or purging cycle is continued intermittently a number of times (4 or 5) to insure that all residual air is removed from the chamber prior to initiation of CVD.

After approximately 15 minutes of cycled purging, the source of argon is shut off by valve 42 and mold 11 is heated to the deposition temperature for tungsten of about 650 C. The pressure applied is 18 inches of mercury (vacuum). Mold 11 is of molybdenum 1 in. long and 0.744 in. at diameter. The tungsten hexafiuoride is caused to fiow at 350 cc. standard per minute, the hydrogen at 1,000 cc. standard per minute, and the carbon dioxide at 20 cc. standard per minute. This results in an amount of carbon dioxide of about 6% by volume standard of the volume standard of tungsten hexafluoride. The deposition time is minutes which results in plating of the mandrel with a tungsten-carbon dispersion of about 0.050 inch in thickness with a carbon content of about 400 ppm. by weight.

After the period noted, the reaction chamber is flushed with argon as before and the mold allowed to cool. After the cooling, the tungsten coated mold is removed from the reaction chamber, and the coated mold is heat treated under a high vacuum of 10- torr (absolute pressure) at a temperature of 2,000 C. for 50 minutes which transforms the columnar tungsten-carbon dispersion to a fine, strong equiaxed grain structure. After such heat treatment, the molybdenum mold is removed from the essentially tungsten coating by dissolving in aqua regia. The resultant crucible is suitable for employment of high temperature chemical reactions, capable of withstanding temperatures of up to about 2400 C. and still be stable. It has a hardness of about 430 dPh Vickers scale and a yield strength of about 35,000 pounds per sq. inch at 1400 C.

As previously pointed out, the invention hereof is not dependent on any particular form of apparatus or flow system for effecting CVD; and as illustrative of another type of apparatus reference is made to FIG. 5 which discloses a system of the type disclosed in assignees aforementioned copending application by Frederick A. Glaski and James R. Humphrey, Ser. No. 645,278, in which the flow of reactants is periodically reversed to avoid the tapering effect of unidirectional flow of the reactants over a mold of relatively long length by plating the mold in layers tapering in opposite direction to compensate for such overall tapering and thereby cause the plated coating to be of substantially uniform thickness.

The mold or mandrel in FIG. is a relatively long molybdenum tube for the formation of relatively long tungsten tubing. The apparatus of FIG. 5 comprises conventional glass tubing 51 of the type described with reference to FIG. 4, clamped in a conventional manner to a suitable support (not shown). Tubing 51 forms reaction chamber 52 through which flow of the reactants is effected, and is 6 ft. long and 1.5 in. inside diameter. Conventional rubber stoppers 53 are removably secured in the respective ends of chamber 52. Mandrel 54 is resistance heated in the chamber by the passage of electric current directly therethrough from solid copper conducting rods 56 passing through the respective stoppers 53, and which are connected to copper cables 57 by means of clamps 58; the mandrel being clamped between the ends of rods 56. To allow for expansion of the parts under heat, the lower rod 56, as shown in FIG. 5, is placed under tension by means of coil spring 59, to thus insure that it can move slightly through the associated stopper as expansion occurs.

For controlling flow of reactant gases into chamber 52, an automatically operated electromagnetically controlled inlet valve 61 is connected to one end of the chamber, and another similar inlet valve 62 is connected to the opposite end. These valves are also connected to manifold tubing 63 for the conduction of the reactant gases, and which has extensions 64 extending through the respective stoppers 53. Manifold tubing 63 is also connected to a source of reactants comprising source 68 of tungsten hexafluoride connected to a conventional fiow meter 69, with a manually operable control valve 71 after flow meter 69. Similarly, a source 72 of hydrogen is connected to conventional flow meter 73 and another manually operable control valve 74. As previously explained, when a CVD apparatus is started, it is usually desirable to purge the same with an inert gas; and for this purpose a source 76 of argon is connected to flow meter 77 and to manually operated valve 78. Also, a source 79 of carbon dioxide, is connected to flow meter 81 and to manually operable control valve 82.

Means is provided for alternately periodically effecting flow of the reactants through the respective ends of reaction chamber 52 comprising tubing 91 connected to a constant source of vacuum provided by vacuum pump 92 connected to gauge 92". Tubing 91 has extensions 93 extending through and gripped by the respective stoppers 53. The exhaust of reactants from one end of chamber 52 is automatically controlled by electromagnetically operated exhaust valve 94; and similarly, exhaust from the opposite end of chamber 52 is controlled automatically by electromagnetically operated exhaust valve 96.

From the preceding, it is seen that periodic reversal of flow of reactants through reaction chamber 52 can be effected by operating the inlet and exhaust valves in such manner that inlet valve 61 and exhaust valve 96 are open when inlet valve 62 and exhaust valve 94 are closed, and vice versa. Any suitable motor timer means may be employed for automatically operating the respective sets 61, 96 and 62, 94 of electromagnetically controlled valves in proper sequence to effect flow of the reactants first in one direction through the reaction chamber and then in the opposite direction sequentially. A suitable form of such mechanism comprising a conventional motor timer which drives cam discs for opening and closing switches in proper sequence, is disclosed in assignees copending application, Ser. No. 645,278.

Example II The following is a typical example of the plating of mandrel 54 in the apparatus of FIG. 5 with a tungsten-carbon dispersed coating. As in Example I, reaction chamber 52 is first purged with argon by flushing the chamber repeatedly for about 15 minutes. After the flushing, mold 54 is heated to the deposition temperature of about 650 C. The pressure applied is 28.5 in. of mercury (vacuum). Mold 54 is a stainless steel tube 5 ft. long and 0.438 in. in diameter. The tungsten hexafluoride is caused to flow at 600 cc. standard per minute, the hydrogen at 3250 cc. standard per minute, and the carbon dioxide at cc. standard per minute. This results in an amount of carbon dioxide of about 18% by volume standard of the volume standard of the tungsten hexafluoride. The total deposition time is about minutes, with a flow in each direction of about 60 seconds, resulting in plating of the mandrel with a tungsten-carbon dispersion of a total overall thickness of about 0.025 in. with a carbon content of about 500 p.p.m.

After the period noted, the reaction chamber is flushed with argon as before and the mold allowed to cool. After cooling, the tungsten coated mold is removed from the reaction chamber, and the coated mold is heat treated in vacuum furnace as in Example I, at a temperature of about 2,000 C. for about 30 minutes which transforms the tungsten-carbon dispersion to a fine, strong equiaxed grain structure. Subsequently, the stainless steel mold is removed mechanically by pulling it out of the coating thereon. The resultant tubing is suitable for use as a sheath for a high temperature thermocouple probe, and is capable of withstanding temperatures of up to about 2400 C. and still be stable. It has a hardness of about 470 DPH Vickers scale and a yield strength of about 31,000 pounds per square inch at 1400 C.

The above examples are merely exemplary because as previously explained, the method hereof is applicable to any type of CVD system. All that is necessary is to introduce a gaseous source of carbon into the reaction chamber simultaneously with the introduction of gaseous reactants for effecting deposit of the refractory metal onto the hot surface of a mold or substrate.

The results have been found particularly advantageous for tungsten wherein a carbon content of about 15 0 to 800 p.p.m. uniformly dispersed throughout the tungsten causes transformation of the columnar grain structure to an equiaxed grain structure upon heat treatment. Because the source of carbon is a gas, advantageously carbon dioxide, delicate control of the amount of such gas with respect to the reactants can be obtained to insure that the carbon content does not become excessive. Otherwise a substantial amount of metal carbide is formed which results in extreme brittleness and weakening of the metal.

We claim:

1. In the chemical vapor deposition of tungsten wherein the tungsten is deposited with an inherent columnar grain structure on a surface of an object from gaseous reactants by application of heat to said object while in a chamber; the improvement of forming an equiaxed grain structure in the tungsten by introducing a gaseous source of carbon into the chamber to disperse a trace amount of carbon in said tungsten sufiicient to prevent grain growth in the resulting equiaxsed grain structure, and below about 800 parts per million by weight to preclude formation of a substantial quantity of tungsten carbide in said tungsten with resultant brittleness, and subsequently'heat treating said columnar grain structure under a non-oxidizing environment at a temperature and time sufficient to convert said carbon containing tungsten to a stable equiaxed grain structure.

2. The method of claim 1 wherein the amount of carbon introduced into said metal is about 150 to 800 parts per million by weight.

3. The method of claim 1 wherein the tungsten is deposited from the tungsten hexafluoride state, and carbon dioxide is the source of carbon.

4. The method of claim 3 wherein the amount of carbon dioxide introduced into the chamber is about 5 to 25% by volume standard of the volume of tungsten hexafluoride.

5. The method of claim 3 wherein the equiaxed grain structure of the tungsten resulting from the heat treatment imparts increased yield strength to the tungsten compared to the columnar structure of pure tungsten, by a factor of at least 25% at about 1400 C.

6. The method of claim 3 wherein the carbon dioxide introduced into the chamber is in an amount to provide about 150 to 800 parts per million by weight of carbon in the tungsten, and after heat treatment the tungsten has a hardness of about 400 to 500 DPH Vickers scale.

7. The method of claim 1 wherein said gaseous carbon source is carbon dioxide or hydrocarbon gas, and said heat treatment is eifected in a furnace under a vacuum in the order of about 10* torr for about 15 to 60 minutes.

8. Chemical vapor deposited tungsten obtained by introducing carbon from a gaseous source thereof into reactants from which the tungsten is deposited with a columnar grain structure, to disperse a trace amount of carbon into said tungsten, and which has a substantially equiaxed grain structure resulting from heat treatment of such carbon containing columnar grain structure under a non-oxidizing environment, the amount of dispersed carbon in said tungsten being suflicient to prevent grain growth in the resulting equiaxed grain structure, and below about 800 parts per million by weight to render said tungsten substantially free of tungsten carbide which would impart substantially brittleness to said tungsten.

9. The tungsten of claim 8 having about 150 to 800 parts per million carbon by weight uniformly dispersed therein.

10. Tungsten of claim 9 having a hardness of about 400 to 500 DPH Vickers scale, and a yield strength of at least about 30,000 lbs. per sq. inch at 1400 C.

References Cited UNITED STATES PATENTS 2,809,140 10/1957 Smeaton 148l3.1 2,976,194 3/1961 Commanday l48-l3.1 3,188,230 6/1965 Bakish et a1. 117107.2X

OTHER REFERENCES Childr, W. 1., et al.: Molybdenum Plating by Reduction of the Pentachloride Vapor, presented at American Society for Metals, October 1950, pp. 1 and 6-10, cited of interest.

Campbell, I. E., et al.: The Vapor-Phase Deposition of Refractory Materials, Electro Chemical Soc. Journal, vol. 96, No. 5, pp. 3 183 25, cited of interest.

ALFRED L. LEAVITT, Primary Examiner W. E. BALL, Assistant Examiner U.S. Cl. X.R. 117107 

